Let's skip test_create_update_server_with_subnet from
heat_tempest_plugin because it is constantly failing on a timeout
'failed to reach UPDATE_COMPLETE status within the required time
The test is working locally so the failure is not caused by any
tempestconf's recent change or by a tempest.conf issue in general.
In order to unblock tempestconf's CI, the patch excludes the test
in python-tempestconf-tempest-devstack-admin-plugins job.